This invention relates to a method of purifying carbon nanotubes by removing coexisting carbon materials of different shapes.
A method for large-scale synthesis of carbon nanotubes is reported in Nature, Vol. 358(1992), No. 6383, pp. 220-222. The method is called a carbon arc plasma discharge method. In an inert gas at a pressure of 200-2500 torr, an arc discharge is made between two carbon rod electrodes by application of a suitable AC or DC voltage (e.g. about 18 V) to thereby produce a carbon plasma. The electric current is about 50-100 A. As the result a carbon deposit forms on the end of one of the two carbon rods, and a core part of the carbon deposit contains a large amount of carbon nanotubes. This core part can easily be separated from a shell part in which no carbon nanotubes exist. Usually carbon nanotubes occupy more than 65 wt % of the core part of the deposit, and the nanotubes coexist with some (less than 35 wt %) carbon nanoparticles which are nanometer-scale carbon particles with polyhedral cage structures. Sometimes a small amount of amorphous carbon also coexists.
Carbon nanotubes have good prospects of becoming valuable materials in the next generation industries ranging from chemical industry to electronical industry. However, for further studies and practical applications it is necessary to find a good method for separating carbon nanotubes from carbon nanoparticles and amorphous carbon. Although it is possible to accomplish the separation by using some of known physical separation techniques such as filtration, chromatography, etc., these techniques are generally troublesome and unsuitable for large-scale purification.